FIG. 1A is a side-view of a SPAR 100. The SPAR 100 is a large un-propelled vessel of generally circular-cylindrical form that is oriented in the sea 102 with its long as 104 vertical. When ballasted, the SPAR 100 exhibits a very deep draft in comparison with its diameter 106 and/or its freeboard 108, thus providing a very stable platform of large volume for offshore petroleum production and storage. SPAR vessels can be positioned and restrained by an elaborate system of moorings, such as moorings 110, which may be anchored to the ocean floor 111.
When beset by ocean currents 112, a SPAR 100 will exhibit substantial drag forces and large scale, long period, shed-vortex induced vibrations ("VIV"). As shown in FIG. 1B, which is a top-view of SPAR 100, VIV is induced when currents 112 travel around the hull of the SPAR 100, forming a down-current pressure gradient 114 and assymetrical circular eddies 116. The assymetrical nature of these eddies 116 causes the SPAR 100 to oscillate orthogonally relative to the currents 112 These oscillations are known as VIV.
A system that has been used to attenuate such VIV is the addition of large scale helical "strakes" 118 to the exterior of the hull of the SPAR 100. While the addition of the strakes 118 may reduce VIV, the strakes 118 actually increase drag and cause the mooring systems 110 to be overwhelmed in the face of currents. This forces the development and use of means other than the strakes 118 to reduce drag and relieve the stress on mooring systems.
FIGS. 2A and 2B show a marine riser 200. Marine risers 200 are used to connect a floating drilling vessel 202 to the ocean floor 204 and to provide a conduit for a drill string and drilling fluids. Like SPAR 100, when beset by ocean currents 206, marine riser 200 will exhibit substantial hydrodynamic drag forces and VIV. Such forces and motions induce mechanical stresses in, and deflections of, the marine riser 200 and its connection 210 to the drilling vessel 202 and connection 212 to the ocean floor 204, which ultimately may result in failure or interference with drilling operations.
Drag and VIV have been reduced by the application of fairings 214 to the marine riser 200. The fairings 214 are enabled passively to rotate about the riser 200 in order to align with the direction of the current 206 to minimize drag. While some drag and VIV reduction is thereby obtained, the procedure for applying and removing fairing segments from riser joints while they are being run and retrieved is lengthy. Slowed riser deployment and retrieval reduces availability and safety of the drilling rig, with important economic consequences. Fairings 214 suffer another disadvantage, in that fairing sections are bulky, expensive, and subject to damage when being deployed through the ocean surface wave zone.
FIGS. 3A and 3B show a semi-submersible drilling vessel 300. Such vessels 300 are often configured as a platform 302 supported well above the ocean surface 304 on submerged longitudinal cylindrical buoyancy pontoons 308. When beset by ocean currents 310, the pontoons 308, being generally bluff, cylindrical objects, exhibit substantial hydrodynamic drag due to flow separation. In order to maintain position relative to the ocean floor 312, such vessels 300 are fitted with a system of moorings 314 and/or powered thrusters 316 to counter the drag forces.
Both moorings 314 and thrusters 316, however, are expensive, and moorings 314 become impractical in very deep water. The presence of mooring winches 318 adds substantially to the topside weight carried by semi-submersible drilling vessels 300. This increase in weight reduces the payload capacity of the vessel 300 and impairs its hydrostatic stability.
Hydrodynamic drag of a semi-submersible drilling vessel 300 can be reduced to a degree by rotating the vessel 300 so that the submerged pontoons 308 are aligned with the direction of current 310 and those located down-current are relatively "shadowed" by those located up-current, as shown in FIG. 3B, which is an end-view of FIG. 3A. Newer designs of semi-submersible drilling vessels are intended to be more azimuthally uniform in their hydrodynamic drag characteristics. This uniformity obviates directional drag reduction.
Boundary-layer-control ("BLC") has been investigated to attain high-lift on aircraft wings and to promote laminar, low friction, flow on wings and elongated bodies. But no such system has been proposed or applied to fluid-submersed bluff bodies to reduce pressure drag and VIV. Unlike aircraft wings, fluid-submersed hulls, such as SPARs, marine columns and risers, and semi-submersible drilling vessels, are generally large, bluff, unstreamlined vertical circular-cylindrical forms or non-circular and/or horizontal cylindrical forms.
The presence of high drag levels and VIV on SPARs, marine risers, and semisubmersible drilling vessels. Drag and VIV may prevent operation of such ocean-deployed vessels, at a high cost to drilling operations. Thus, the inability to substantially reduce drag and VIV may have high economic costs.
Accordingly, while various systems and methods exist for reducing VIV and hydrodynamic drag in fluid-submersible objects, no such system or method reduces both VIV and hydrodynamic drag to a substantial degree. Moreover, while BLC has been applied to streamlined aircraft wings to attain high-lift, BLC has not been designed or applied to fluidsubmersed hulls, such as SPARs, marine risers and columns, and semi-submersible drilling vessels. Accordingly, a need exists for a system and method for reducing VIV and hydrodynamic drag in fluid-submersed hulls and for thereby preventing suspension of drilling operations and other functions in the presence of currents in the fluid.